Conventionally, there has been widely performed a technology of subjecting a heat-shrinkable multilayer film to bag-making into a bag or a pouch with a bag sealer and filling up the bag or pouch with a content, or to automatic packaging of a content directly or on a tray with such a heat-shrinkable multilayer film while concurrently subjecting the film to bag making, for packaging and processing products, inclusive of food, such as uncooked meat, ham, and sausage, and other products. Such a packaging film is required of various properties, and an excellent balance among various properties, inclusive of: heat-shrinkability, strength, heat resistance, high-temperature creep-resistance, formability, adaptability to various packaging modes, gas-barrier property, water vapor barrier-property, and transparency.
As a packaging material satisfying various characteristics as described above and suitably used especially as a heat-shrinkable packing material required of a strength, there has been proposed a heat-shrinkable multilayer film including a polyamide resin layer as a principal resin layer. For example, Patent document 1 shown below has proposed: “a heat-shrinkable multilayer film, comprising: an outer surface layer (a) comprising a thermoplastic resin, a first intermediate layer (b1) comprising a polyamide resin, a second intermediate layer (b2) comprising saponified ethylene-vinyl acetate copolymer, and an inner surface layer (c) comprising a polyolefin resin, wherein the heat-shrinkable multilayer film has a hot-water shrinkability at 90° C. of 3-45% in each of longitudinal and transverse directions, and the above-mentioned polyamide resin is a mixture of 60-90 wt. % of an aliphatic polyamide resin having a melting point of at least 210° C., and 10-40 wt. % of an aromatic polyamide resin comprising isophthalic acid and terephthalic acid as principal acid components.
On the other hand, there is also known a method of forming a base material by deep draw forming (or simply, deep drawing), which base material is capable of allowing automatic packaging more easily in combination with an ordinarily flat lid material instead of a bag-making step into a bag or pouch. As a film for deep draw forming, an unstretched multilayer film is generally known to be excellent in deep drawability but, including that of a polyamide resin-based multilayer film, an unstretched multilayer film provides only a low strength of film after deep drawing, and is thus accompanied with problems such that the film is liable to be broken, the film thickness has to be increased remarkably, and the film shows an insufficient heat-shrinkability or is liable to generate wrinkles after the shaping.
The polyamide resin-based heat-shrinkable multilayer film of the above-mentioned Patent document 1 has excellent heat-shrinkability and strength, shows particularly excellent performances as bag-making packaging materials, such as a bag and a pouch, and also solves the problems of the unstretched multilayer film described above as a multilayer film for deep drawing, but still involves a problem that it is liable to be broken to lose practical applicability to a high degree of deep drawing, especially to such a deep drawing performed by using a deep drawing mold having a rectangular opening with a longer side and a shorter side and requiring a shorter side-basis draw ratio (D/L) exceeding 0.6 as determined by a maximum draw depth (D)/shorter side (L) of the mold, and/or a deep drawing using a mold having a sloped bottom face. For example, such a high degree of deep drawing is required for packaging of a content material, such as sausage, having a large length/width ratio, a wedge-shaped content material, such as a cheese piece, or packaging of a cut product, such as cut salami, ham or sausage, for providing a package of which the commodity value is liable to be impaired if the seal part is disposed on the cut face.
Further, Patent document 2 shown below has proposed “a stretch-oriented multilayer film, comprising: a surface layer (a) comprising a thermoplastic resin, an intermediate layer (b) comprising a polyamide resin, and a surface layer (c) comprising a sealable resin, wherein the multilayer film shows an impact resistance of at least 1.5 Joule at a conversion thickness of 50 μm at −10° C.” The multilayer film has taken note of a deep drawability in addition to the low-temperature packaging performance. However, the film of Patent document 2 still fails to satisfy a high degree of deep drawability as represented by a shorter side-basis draw ratio (D/L) exceeding 0.6.